cmd: change the prompt in lua

This keeps the PROMPT variable as is and changes the prompt to the cmder style
in the clink code.

This has two advantages:

* opening a cmd in a cmder session will now show the old prompt code instead of
  a ugly raw prompt without the replacements. This led to ugly output when a
  batch file echoed their content (e.g `conda build recipe/`).

* when a command rewrites the prompt (e.g. an activate in a virtualenv), these
  command sometimes simply overwrites the PROMPT so that the cmder enhancements
  were not anymore in place. Now we simply don't care and overwrite it with our
  stuff in the clink part. This might mean that a user has to install a lua
  script so that e.g. conda environments are visible on the prompt.

vendor/init.bat

@@ -17,10 +17,6 @@ if not defined CMDER_ROOT (
 :: Remove trailing '\'
 if "%CMDER_ROOT:~-1%" == "\" SET "CMDER_ROOT=%CMDER_ROOT:~0,-1%"
 
-:: Change the prompt style
-:: Mmm tasty lamb
-prompt $E[1;32;40m$P$S{git}{hg}$S$_$E[1;30;40m{lamb}$S$E[0m
-
 :: Pick right version of clink
 if "%PROCESSOR_ARCHITECTURE%"=="x86" (
     set architecture=86